We have investigated the influence of plasma pretreatment in Various gases (Ar, NH3, N-2) on the adhesion of plasma-deposited silica layers to polypropylene (PP). UV-visible ellipsometry measurements enabled us to detect crosslinking at the polymer surface. All treatments, and especially the Ar-diluted N-2 treatment, induced crosslinking in the surface layer. The adhesion of silica to PP, as measured by the micro-scratch test, was improved by every treatment. In the case of Ar, the adhesion improvement is attributed to crosslinking, and possibly to surface activation by radical formation. In addition, surface functionalization was studied by in situ IR ellipsometry, X-ray photoelectron spectroscopy, and contact angle measurements. N-2 plasma treatments induced a significant nitrogen grafting (17.5 at.% concentration after dilute N-2 treatment, in the form of amines and imines), leading to an increase in both the electron-acceptor and the electron-donor components of the surface free energy. In contrast, dilute NH3 plasma treatment seemed to result in weak bonding of the nitrogen atoms to the surface, as deduced from a comparison of measurements performed in situ after this treatment, after aging, and after an additional SiH4 plasma treatment. The dilute NH3 treatment, performed after Ar treatment, did not further improve the adhesion of silica to PP, while an additional improvement was obtained with dilute and pure N-2 treatments. This may be attributed to C-N-Si bond formation or to acid-base interactions between the nitrogenated polymer surface and silica.